During the fabrication of semiconductor, especially the process of photolithography on wafers, wafers are labeled with markings in order to identify and distinguish wafers. The marking may be done by ink labeling or laser labeling on the front side or back side of a wafer, but markings on the back side of the wafer tend to be worm off and become unreadable. Thus, labeling markings on the front side of wafers is a better solution.
As markings are labeled on the front side of wafers, conventional techniques typically perform photolithography for the markings during the photolithography of each layer. The thickness of the wafer in areas in the vicinity of the marking tends to become uneven after processes of photolithography and washing for subsequent layers. When the same mask is used for photolithography, the pattern projected unto that region tends to be defocused, resulting in high tendency of failure in the chips made from around that region of the wafer.
A conventional approach is to trim the scope of exposure of photolithography for the marking so that it is closer to the edge of the wafer in order to reduce the area of defocusing. Nevertheless this approach does not completely solve the problem of defocusing.